There have been too many surprising flops and improbable hits for anyone to construct a unified theory of what makes a good television drama. But a significant number of the medium's most notable fictions have involved people in the wrong jobs. Captain Mainwaring should never have got the chance to command even a part-time army platoon; Basil Fawlty was ill-advised to take up catering; Del Trotter lacked the street wisdom for being a Peckham fence; David Brent was remarkable for alternating two professions that didn't suit him, middle management and showbusiness.

Ugly Betty, the hit US drama launching here on Channel 4 next month, has some way to go before it belongs in the company of Dad's Army, Fawlty Towers, Only Fools and Horses and The Office. But, promisingly, it revolves around counter-intuitive recruitment. Ugly Betty is the story of Betty Suarez (America Ferrera), a gauche Latino woman who lands a post as assistant on a top US fashion magazine, Mode, despite a coiffure that knows only bad-hair days, dental braces that resemble the fences at Guantánamo Bay, eyeglasses issued by the Colombian National Health Service, and a wardrobe bought from discount brochures.

In fact, as viewers learn in episode one, it was Betty's apparent disadvantages that got her the job. The magazine's editor-in-chief, Daniel Meade, has a history of being distracted from his duties through seducing his underlings. Daniel's dad and Mode's owner, Bradley Meade, sends in Betty to guarantee that the boss's eye will stay on the bottom line.

Premiered by ABC, Ugly Betty was the only show among the autumn 2006 launches from US networks that worked for both reviewers and viewers. (Studio 60 on the Sunset Strip, NBC's new series from West Wing creator Aaron Sorkin, has achieved only cult appeal.) Already, Betty has been nominated for Golden Globe and Writers' Guild awards for best new drama.

In television, as in other industries, America has an export rather than import mentality, but Ugly Betty is a Hollywood version of a 1999 hit in Colombia. Yo Soy Betty, La Fea (I am Betty, the Ugly One) was a telenovela, or soap opera with season-long storyline - a genre popular with viewers in Latin and South America.

Ugly Betty began with two obvious obstacles: a media setting (traditionally considered too niche for peak-time), and a central actress who, at least when dressed in character, was useless for the soft-focus covershots favoured by TV magazines and billboard posters. A third drawback soon emerged: The Devil Wears Prada, the Meryl Streep movie in which Anne Hathaway plays a misplaced intern on a glamour mag, was due to be released shortly before the series aired. If Prada was loved, Betty would seem redundant; if the movie failed, the TV show could resemble another spoonful from a jar with a nasty taste. But the series overcame both problems. While audience resistance to Studio 60 on the Sunset Strip seems to stem from its setting within the television business, Ugly Betty achieves a balance between plotlines that arise from the magazine background (in the second episode, Betty loses the "book", the proofs of the next edition) and a more generalised fairytale about a misfit, even extending to a fairy godmother, a kind seamstress played by Ashley Jensen (Maggie in Ricky Gervais's Extras).

The other two matters - the lead character's appearance and the possible clash with The Devil Wears Prada - are closely linked. Although Anne Hathaway is supposed to be playing a fashion disaster, this amounts to little more than one of Hollywood's most beautiful performers wearing a pullover for a short time. But 10 episodes into the US run of Ugly Betty, America Ferrera still looks like the "before" part of the advert. In interviews, Ferrera refers to her daily hour's uglification before shooting, the make-up trailer being turned to make-down. Still, she is allowed to show that Ms Suarez is savvy and even sexy beneath her anti-fashion mask. For this reason, although the pilot episode of Ugly Betty had the misfortune to share several jokes and one plotline with The Devil Wears Prada, it immediately seemed a more mature and surprising version of the Cinderella at Vogue storyline.

Most of American television's recent hits have been structured as "television novels", a phrase first floated by Steve Bochco, creator of LA Law and NYPD Blue, to describe Murder One, his 1995 legal procedural following a single case across a 20-week season, rather than the traditional plotlines resolved within a single episode or, for high days and holidays, a two-parter.

The American audience wasn't ready at that time for the 20-parter but, a decade on, the stand-out dramas - 24, Desperate Housewives, Lost, The Sopranos and now Ugly Betty - all extend a single plot arc across half a year of programmes. HBO's brilliant police procedural The Wire employs novelists, including George Pelecanos and Dennis Lehane, as scriptwriters to achieve a long, chaptered narrative.

Ugly Betty is typical of the US television novel in that, while each episode has a new central plotline, the major stories are continuous: Betty's relationship with Daniel, and a possible mystery involving the death of a previous editor. The paradox of this new style of television, though, is that series aspiring to tell their stories over a longer time often involve dramatic scenarios with limited potential to develop. In Lost, plane-crash passengers are stranded on an island; in Ugly Betty, a young woman has got the wrong job. These are situations so static they initially seem more suited to a sitcom, the most repetitive kind of fiction, rather than a form that seeks to bring the expansiveness of the Victorian novel to the screen. So, just as Lost has always suffered a tension over how long its survivors could hang on without being found, Ugly Betty is haunted from its opening episode by the question of when the central character will submit to a makeover. But, in resolving these strains, both shows would destroy their central premise.

Lost has got round the problem by becoming more supernatural and science-fiction-oriented, but it's hard to imagine Ugly Betty can sustain itself by having Ms Suarez suddenly clamber through a hatch in the restrooms. Distinctive and intriguing as the opening episodes are, the show may, like its heroine, lack long legs.

The concern over television adopting the novel as a model for storytelling is that very few works of prose fiction merit a sequel: the Rabbit books of John Updike and the Smiley novels of John le Carré are exceptions. In contrast, the economics and viewing patterns of television demand that a popular show returns for as many series as possible. The creators of Life on Mars - the closest British TV has recently come to the "television novel" - have sensibly decided to conclude the story in two series. The point about novels is that they have full stops.